Mouse selection, copy/paste, yield_to_user fixes

- Mouse text selection with highlight rendering in panes
- OSC 52 clipboard copy on selection, middle-click paste via tmux buffer
- Bracketed paste support (Event::Paste)
- yield_to_user: no tool result appended, ends turn immediately
- yield_to_user: no parameters, just a control signal
- Drop arboard dependency, use crossterm OSC 52 + tmux for clipboard

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/agent/tools/mod.rs

@@ -246,10 +246,7 @@ pub fn summarize_args(tool_name: &str, args: &serde_json::Value) -> String {
                 entry.to_string()
             }
         }
-        "yield_to_user" => args["message"]
-            .as_str()
-            .unwrap_or("")
-            .to_string(),
+        "yield_to_user" => String::new(),
         "switch_model" => args["model"]
             .as_str()
             .unwrap_or("")